The present invention relates to a manufacturing method of an excess lithium type manganese-containing spinel compound having a low specific surface area for used in an active material of a positive electrode for a lithium ion secondary cell in which an intercalation compound, such as metallic lithium, lithium-carbon (lithium-graphite) or the like, serves as an active material of a negative electrode.
In addition to LiNiO2, LiCoO2 and LiMn2O4 may be used as an active material for a positive electrode in a 4-volt, high-energy density type of lithium secondary cell. Some cells using LiCoO2 as the active material of the positive electrode have been placed on the market. However, cobalt is not suitable for mass-production along with the popularization of such cells due to its limited resource and high-cost. In view of resource and cost, manganese compounds have promise as a material of the positive electrode. Manganese dioxide available for a raw material of the active material is mass-producing for materials of dry cells.
Spinel-structured LiMn2O4 has a weak side in that its capacity is degraded in the course of repeated charge/discharge cycles. Various approaches have been attempted to improve this drawback, such as addition of Mg, Zn or the like (Thackeray et al. Solid State Ionics, 69, 59 (1994)) or addition of Co, Ni, Cr or the like (Okada et al. Battery Technology, Vo. 5, (1993)), and its effectiveness has been clarified. However, Mn is materially dissolved in an electrolyte in high-temperature operation at 50xc2x0 C. or more, and thereby the capacity is undesirably degraded in parallel with the charge/discharge cycles. Thus, it is difficult to maintain sufficient cycle life of the positive electrode only by simply doping the aforementioned metals.
The present invention has been embodied with a view to the aforementioned problems. Thus, it is an object of the present invention to provide a manufacturing method of a spinel-type manganese oxide for a lithium secondary cell, capable of improving high-temperature cycle characteristics by maintaining the features of lithium-rich spinel structure in which Li exists at the 16d site excellent in the cycle characteristics and reducing the specific surface area of. the lithium-rich spinel structure.
According to the present invention, there is provided a manufacturing method of a spinel type manganese oxide for a lithium ion secondary cell, comprising the steps of pre-firing a mixture of lithium salt including lithium carbonate, manganese oxide, and heterogeneous metal; firing said mixture at 900 to 1200xc2x0 C. to form a raw material; adding in said raw material at least one of crystal growth accelerators selected from the group consisting of lithium hydroxide, lithium sulfide and a mixture thereof; and firing said resulting compound at 750 to 850xc2x0 C. to form an excess lithium heterogeneous metal-doped spinel compound having a BET specific surface area of 0.5 m2/g or less.
In a specific embodiment, the doped metal may be a spinel compound substituted by Ni, Co, Fe, and Cu. Further, the excess lithium heterogeneous metal-doped spinel compound may have a composition of Li1+xMn2+yxe2x88x92xMyO4xe2x88x92zSz, where 0.01xe2x89xa6xxe2x89xa60.10, 0.01xe2x89xa6yxe2x89xa60.20, 0 less than zxe2x89xa60.05.
Other features and advantages of the present invention will be apparent from the detailed description.
It has been checked out that stoichiometric LiMn2O4 was transformed into a lithium-rich spinel compound having a low capacity as repeated charge/discharge cycles and gradually showed a stable capacity. It is a logical conclusion that lithium-rich spinel provides better cycle characteristics, and this has been experimentally verified (Yoshio et al. J. Electrochem. Soc., 143, 625 (1996)). However, as Li/Mn ratio is increased, the capacity is degraded and the availability for the material of the positive electrode will be eventually gone off. As described above, the heterogeneous metal-dope is also effective to improve the cycle characteristic. In this case, lager capacity may be obtained by forming the 16d site with Li, Mn, and M (Ni, Co, Fe, Cr, and Cu) as compared with the case of simply constructing with Li and Mn.
On the other hand, when the active material is dissolved in a high-temperature electrolyte, reduced reaction area obviously allows the dissolved amount of manganese to be lowered so that the high-temperature cycle characteristics may be enhanced. Otherwise, improved high-temperature cycle characteristics may be expected by compounding the positive electrode through high-temperature firing capable of enhancing sintering and crystal growth. However, when the firing temperature is simply increased, an oxygen-defect type spinel compound is created which is inferior in the high-temperature cycle characteristics due to a voltage plateau around 3.3 V.
It has been discovered that a material of a grown crystal structure having a narrow line width of XRD diffraction patterns and 0.5 m2/g or less of specific surface area evaluated by BET method was provided by mixing a pre-mixed spinel compound with lithium hydroxide and then firing to form an excess lithium spinel compound. This proves that the lithium hydroxide acts as a crystal growth accelerator. It has also been verified that the same effect was yielded by lithium sulfide and a mixture of lithium sulfide and lithium hydroxide as well as lithium hydroxide. This result has been applied to a method for manufacturing a heterogeneous metal-doped spinel compound excellent in the high-temperature characteristics.
While a spinel compound having a smaller specific surface area may be obtained by compounding through firing at 900xc2x0 C. or more a heterogeneous metal-doped spinel compound in which the 16d site is formed of Li, Mn, and M (Ni, Co, Fe, Cr, and Cu), an oxygen-defect spinel structure is generally created and thereby the high-temperature cycle characteristics is deteriorated. Generally, when a pure spinel compound formed only of manganese is fired at about 750xc2x0 C. of firing temperature, 0.5 to 1% of the 16d site involves cation defect (Yoshio et al. J. Power Sources, 77, 198 (1999)). In contrast, according to the method of the present invention, the spinel compound is compounded at 900xc2x0 C. or more of high temperature so that no cation-defect type spinel compound may be provided. When this raw material is re-fired after adding lithium hydroxide, lithium sulfide, or a mixture thereof as a crystal growth accelerator, an excess lithium type spinel structure having excellent cycle characteristics, a large crystal having a grown crystal structure, and small specific surface area are desirably provided.
In particular, if lithium hydroxide and lithium sulfide are used together as a crystal growth accelerator, a more desirable compound is provided with a smaller surface than that resulting from using only lithium hydroxide and with sulfur substituted for a part of oxygen.